Stable white thermal control coatings are used on radiators for a variety of missions. In orbital environments where surface charging occurs, such as polar, geostationary, or gravity-neutral orbits, these coatings must adequately dissipate charge buildup. Most white pigments do not dissipate electrical charge without a dopant or additive. The two most commonly used dissipative thermal coatings (Z93C55 and AZ2000) rely on indium hydroxide or tin oxide as charge dissipative additives.
Work previously conducted at Goddard Space Flight Center (GSFC) sought to encapsulate white coating pigments with indium hydroxide and indium tin oxide (ITO), which is a ternary composition of indium (In), tin (Sn), and oxygen (O) in varying proportions, through sol gel and wet chemistry processing. In these cases, ITO formed locally on a macroscopic scale due to seeding. Thus, ITO crystal formation on the boundaries of the pigment grains and thickness and dispersion throughout the coating were difficult to control, and thicknesses of at least 50-70 nm resulted. Despite improved surface resistivity, the optical properties of the pigment suffered and the resulting coating solar absorptance was higher than the un-doped versions.
Indeed, such charge dissipating additives impact the optical properties and stability of the coating and reduce the efficiency of the thermal design (i.e., reducing reflectance). The end-of-life design properties of the coatings are thus degraded as compared to their un-doped versions, resulting in larger, heavier radiator systems and more complex designs. Accordingly, an improved approach to dissipating charge for thermal control pigments may be beneficial.